N-nitroso-N-hydroxylamine salts, such as N-nitroso-N-phenylhydroxylamine aluminum complexes (also referred to generally as NPAL), are used as polymerization inhibitors or stabilizers in a variety of polymerization systems. For example, NPAL can be used as a stabilizer in free radical polymerization systems (such as used to polymerize olefins, unsaturated polyesters, styrene, acrylates, and the like). NPAL acts to prevent premature polymerization and the attendant problems of increased viscosity/gelling, loss of reactivity of the monomer/oligomer, etc. NPAL and similar compounds can also be used in other types of polymerizations systems, such as those initiated by exposure to radiation (such as UV curable systems), thermally initiated polymerization systems, and others.
Despite its usefulness as a polymerization inhibitor, there are problems associated with the use of NPAL. Specifically NPAL and other N-nitrosohydroxylamine compounds are unstable and undergo degradation, as manifested by discoloration and formation of a precipitate in some instances. The precipitate which is formed is insoluble in all polymerization systems causing turbid mixtures. In addition, the degradation can result in a loss of effectiveness of NPAL as a polymerization inhibitor. Further, customers are reluctant to use the product once it darkens, and the product cannot be readily used in color sensitive applications.
Prior attempts to stabilize NPAL, e.g., to minimize the extent and rate of degradation, include storing the product in a dark (amber) container under an inert atmosphere (nitrogen blanket). In addition, typically NPAL is shipped and stored under cold conditions, generally being refrigerated until used. This, however, can be costly.
U.S. Pat. No. 4,898,976 to Varwig describes the degradation of such compounds, and in particular cupferron (the ammonium salt of N-nitroso-phenylhydroxylamine) as a result of the instability of solutions thereof in the presence of air. To address the degradation of cupferron, the '976 patent reports the use of an ethanolamine salt of N-nitrosophenylhydroxylamine.